Recently, an amount of information to be transmitted over the optical communication continues to increase. In order to deal with the increased amount of information, countermeasures are taken, such as increasing a signal speed and/or increasing the number of channels employing a wavelength multiplexing communication technique. Among these, a next-generation 100 Gbps digital coherent transmission technique, which is capable of increasing a signal speed, employs a polarization multiplexing technique in order that an amount of transmittable information per unit time is doubled. According to the polarization multiplexing technique, different pieces of information are respectively carried by two types of polarized waves whose electric fields are orthogonal to each other. However, a modulation method employing the polarization multiplexing technique requires an optical modulator haying a complex structure. This results in problems such as an increase in a device size and an increase in cost.
In order to address these problems, Non-Patent Literature 1 discloses an optical modulator including a substrate-type optical waveguide that includes a core made of silicon and has advantages such as an easy manufacturing process, a smaller optical element thanks to high-density integration, and a reduction in manufacturing cost due to use of a larger-diameter wafer.
The optical modulator employing the polarization multiplexing technique includes a polarization rotator (hereinafter, abbreviated as “PR”) and a polarization beam combiner (hereinafter, abbreviated as “PBC”). FIG. 24 is a block diagram illustrating a configuration of PR 6, whereas FIG. 25 is a block diagram illustrating a configuration of PBC 7. PR 6 includes an input port and an output port. PR 6 receives a TE polarized wave via the input port, converts the TE polarized wave into a TM polarized wave, and outputs the TM polarized wave via the output port. PBC 7 includes a first input port, a second input port, and an output port. PBC 7 multiplexes a TE polarized wave inputted to the first input port and a TM polarized wave inputted to the second input port, and then outputs, via the output port, the TE polarized wave and the TM polarized wave thus multiplexed. By using PR 6 and PBC 7 in combination, it is possible to provide a polarization multiplexing waveguide.
FIG. 26 is a block diagram illustrating a configuration of an optical modulator 8 including a polarization multiplexing waveguide 9. Namely, the optical modulator 8 is an optical modulator employing the polarization multiplexing technique. The optical modulator 8 includes the polarization multiplexing waveguide 9 including PR 6 and PBC 7, a first phase modulator for modulating a TE polarized wave, and a second phase modulator for modulating a TM polarized wave.
Individual TE polarized wave light beams inputted to the respective phase modulators are modulated by independent electrical signals. Further, different pieces of information are superimposed thereon. The polarization multiplexing waveguide 9 is disposed so as to follow the phase modulators. Out of the two TE polarized waves inputted to the polarization multiplexing waveguide 9, the TE polarized wave inputted to the first input port is converted into a TM polarized wave. Then, the TM polarized wave and the TE polarized wave inputted to the second input port are multiplexed, and a resultant of the multiplexing is outputted. Thus, by employing the polarization multiplexing waveguide, it is possible to use the first phase modulator and the second phase modulator, which have similar configurations.
Herein, the TE polarized wave refers to a mode including, as a main component, an electric field component that is in a direction (hereinafter, referred to as a “width direction” or an “x-direction”) horizontal to a substrate, in a plane perpendicular to a traveling direction of light in the substrate-type optical waveguide. Particularly, a TE polarized wave having a maximum effective refractive index is called a “TE0 polarized wave”. Meanwhile, the TM polarized wave refers to a mode including, as a main component, an electric field component that is in a direction (hereinafter, referred to as a “height direction” or a “y-direction”) perpendicular to the substrate, in the plane perpendicular to the traveling direction of light in the substrate-type optical waveguide. Particularly, a TM polarized wave having a maximum effective refractive index is called a “TM0 polarized wave”. The TE0 polarized wave and the TM0 polarized wave are confined in the waveguides strongly more than any other TE polarized waves and any other TM polarized waves. For this reason, the TE0 polarized wave and the TM0 polarized wave are waveguide modes widely used for the substrate -type optical waveguide element.
Non-Patent Literatures 2 and 3 disclose a polarized wave beam splitter. The polarized wave beam splitter disclosed by Non-Patent Literatures 2 and 3 may also serve as a polarization beam combiner when an input and an output thereof are reversed.
A substrate-type optical waveguide element according to Non-Patent Literature 2 is constituted by two elements, specifically, asymmetric Y-branching (corresponding to an asymmetric Y-junction of Non-Patent Literature 2) and a tapered waveguide having a rib waveguide structure (corresponding to a taper of Non-Patent Literature 2). In a case where the substrate-type optical waveguide element of Non-Patent Literature 2 serves as a polarization beam combiner, the asymmetric Y-branching converts, into a TE1 polarized wave, one of two TE0 polarized waves that are spatially divided, and multiplexes the TE1 polarized wave thus converted and the other one of the two TE0 polarized waves. Here, the TE1 polarized wave refers to a waveguide mode having a second maximum effective refractive index among the TE polarized waves. The tapered waveguide converts, into a TM0 polarized wave, only the TE1 polarized wave out of the TE1 polarized wave and the TE0 polarized wave multiplexed by the asymmetric Y-branching. In this manner, the element according to Non-Patent Literature 2 serves as the polarization multiplexing waveguide.
A substrate-type optical waveguide element according to Non-Patent Literature 3 is constituted by two elements, specifically, an adiabatic conversion coupler (an adiabatic coupler of Non-Patent Literature 3) and a tapered waveguide having a rib waveguide structure (a bi level taper of Non-Patent Literature 3). In a case where the substrate-type optical waveguide element of Non-Patent Literature 3 serves as a polarization beam combiner, the adiabatic conversion coupler converts, into a TE1 polarized wave, one of two TE0 polarized waves that are spatially divided, and multiplexes the TE1 polarized wave thus converted and the other one of the two TE0 polarized waves. Here, the TE1 polarized wave refers to a waveguide mode having a second maximum effective refractive index among the TE polarized waves. The tapered waveguide converts, into a TM0 polarized wave, the TE1 polarized wave out of the TE1 polarized wave and the TE0 polarized wave multiplexed by the adiabatic conversion coupler. In this manner, the element according to Non-Patent Literature 3 serves as the polarization multiplexing waveguide.